The invention relates to a cooling system for a motor vehicle.
Modern motor vehicles already have, on account of increasing levels of engine power and an increasing number of auxiliary units, a high overall thermal output, which requires high cooling capacities in order to dissipate heat by means of heat exchangers. Here, the often limited installation space for a heat exchanger or combination of heat exchangers arranged in the relative wind has in the meantime been completely used up. The possible flow of ambient air is generally improved by means of ever more powerful fans which are usually arranged on the suction side of a main cooler. Overall, the maximum cooling capacity by means of heat exchangers or heat exchanger packs arranged in the relative wind or at the end side of a motor vehicle has been largely exhausted.
This situation clashes with ever stricter emissions standards which are impending in the coming years primarily in Europe and the USA. In order to meet said emissions standards in particular in the case of diesel engines, though fundamentally also in spark-ignition engines and new engine concepts such as for example HCCI, there are numerous proposals to reduce the emissions, in particular of nitrogen oxides, by means of at least partial exhaust-gas recirculation into the combustion tract of the engine. Said exhaust-gas recirculation is only expedient if the exhaust gas is previously cooled. For this purpose, substantially liquid-cooled heat exchangers have been proposed, with the cooling liquid usually being connected to the main cooling circuit of the internal combustion engine. In this way, high levels of thermal output, which can reach up to over 100 kW, are introduced into the main cooling circuit of the internal combustion engine by means of the exhaust-gas recirculation.
In addition, in exhaust-gas recirculation, there is the fundamental problem that the power and pollutant emissions of the engine are all the better the cooler the intake-side gases are. Here, the cooling of the recirculated exhaust gases by means of a liquid heat exchanger arrives at theoretical limits, since at least when using main engine coolant, the temperature of the secondary medium which is to be cooled lies in the region of 100° C.
In addition to the problem of cooling recirculated exhaust gases, there are also increasing problems in cooling charged fresh air. In the meantime, multi-stage charging systems have been developed, with fundamentally the efficiency and power-to-weight ratio of an internal combustion engine being improved by means of high exhaust-gas charging. The high fresh gas temperatures generated during the exhaust-gas charging must however be cooled. In the known arrangements of a charge-air cooler in a structural unit with the main cooler, which is arranged in the relative wind, of a vehicle, the attainable cooling capacity is limited on account of the use of the same air flow of ambient air. Said cooling capacity is ultimately limited by the possibly design-related delimitation of the vehicle end face or of the air inlet cross sections.
DE 102 03 003 A1 describes a cooling system for a motor vehicle in which a part of the exhaust gases of the internal combustion engine are introduced into a charged fresh air flow, with the recirculated exhaust gases first being cooled by means of a liquid heat exchanger which is provided with a bypass. Here, the liquid heat exchanger is connected to the main cooling circuit of the internal combustion engine.